It is well known to use hydraulic binder compositions in the construction field for making the infrastructure of buildings, works of art, apartment buildings or articles such as paving slabs or boards and tiles, and as adhesives or jointing compositions, particularly to adhere tiles or ceramics in general, vertically or horizontally, to plane surfaces of different kinds, such as to concrete, plywood or brick surfaces.
In the present text with the term “hydraulic binders” we designate mineral substances that in the presence of water harden because of hydration chemical reactions and can bind other materials together.
Hydraulic binder compositions that mainly consist of cement mixed with a variable amount of sand and possibly gypsum are generally used to prepare tile adhesives, mortars, concrete, cement plaster, finiture plaster, self-levelling flooring; hydraulic binder compositions based on gypsum are generally used to prepare gypsum plasters and joint compounds.
Immediately prior to use, a proper amount of water is added to the dry hydraulic binder composition, making it a workable paste and allowing the shaping of articles or the application on various surfaces.
The curing of the thus obtained hydraulic binder paste (from now on “paste”), also commonly referred to as “setting”, begins as soon as the composition is admixed with water and will result in its complete hardening and its exhibiting the necessary physical and mechanical features.
Setting is a rather complex chemical process that leads to polymeric inorganic structures whose strong reciprocal interactions result in the formation of solid and strong masses.
In the setting process, many features are of importance and influence not only the speed at which setting occurs but also its final effectiveness, i.e. its solidity. Among these features of fundamental importance are the content of water and the capability of the composition to retain the proper amount of water during the whole setting process. It is important that the paste retains sufficient water until all the desired physical characteristics are obtained.
In practice, most of the surfaces onto which pastes are generally applied are porous and absorbent and they absorb water from the paste in the area of contact, thus creating defects in the setting which may at some point in time result in defects of the adhesion and of the mechanical properties of the hardened composition.
Another problem that may be encountered during the application of pastes that is related to the capability of the paste to retain the proper amount of water during the whole setting process is a too rapid hardening that prevents the adjustment of the laid strata or of the shaped articles. This problem is called “lack of open time” and/or “lack of adjustability time”.
Yet another problem occurs when the content of water becomes excessive, even if only locally or because of a lack of homogeneity of mixing. In such cases, setting becomes too slow due to a too flowing mixture, working time grows longer and the resulting application becomes imprecise and difficult.
Another issue in the handling of pastes is the fact that water in the mixture acts as a lubricant for the solid particles when paste is spread on the surface of the object to which it is to be applied. The proper amount of water gives to the mixture the “pastiness” or “creaminess” suitable for a uniform, homogeneous and easy laying. The rheological characteristics of the final mixture are very important and they depend on the kind and on the amount of the different components in the mixture.
The rheology of a mixture of sand, cement and water alone, or of gypsum and water alone renders them unsuitable for use as pastes because of the lack of the above-mentioned characteristics, and more generally, because of their poor processing.
To overcome all these problems, additives are used in the formulation of hydraulic binder compositions acting as retention agents and rheology modifiers. These additives are generally synthetic or semi-synthetic polymers, usually chemically modified natural polymers, exhibiting the specific characteristic of bonding and coordinating a large amount of water once they are dissolved in water.
These products, and among these in particular cellulose ethers, are highly purified products whose preparation requires many sophisticated and complex purification steps. They are rather expensive products.
In literature many mixtures are described for use as rheology modifier and retention aid in pastes, such as in U.S. Pat. No. 6,706,112, U.S. Pat. No. 4,028,127, EP 235513, U.S. Pat. No. 5,432,215, and U.S. Pat. No. 4,487,864, in which mixtures are also described whose components show synergic effects.
In particular, U.S. Pat. No. 6,706,112 discloses cementitious mortar additives including at least a hydroxyalkyl guar ether having a molar substitution of from about 0.7 to about 3 which is able to impart to mortars very good water retention and an initial adhesion as good as the adhesion of mortars that include cellulose ethers. Nonetheless, it would be desirable in the art of preparing cement and/or gypsum pastes to provide additives that further improve the water retention properties and, as a consequence, the whole processing of such pastes.
Surprisingly, it has now been found that highly substituted hydroxypropyl guar ethers that further comprise a certain amount of relatively short hydrophobic unsubstituted alkyl chains impart improved water retention and processability to cement and/or gypsum pastes.
Hydrophobic hydroxypropyl guar derivatives bearing C10-C32 hydrophobic substituents have been described in the patent literature, by way of example in U.S. Pat. No. 4,870,127 and U.S. Pat. No. 4,960,876; they are said to be suitable for use in many industrial fields, such as in the manufacture of paper coatings and sizings, adhesives, liquid detergents, emulsions used to make polishes, cleaners and lattices, in compositions for textile printing and dyeing, and as textile binders and adhesives, in water borne coatings, as suspending agents in agricultural sprays and as suspending agents for pigments and inks, in the photographic processing and in the manufacture of ceramics, in cosmetics, in the general fields of mining, explosives and oil stimulation.
U.S. Pat. No. 4,870,127 also reports the synthesis of a hydroxypropyl guar derivative comprising a C6-alkyl chain.
U.S. Pat. No. 7,355,039 describes the use of glyoxalated purified hydrophobic hydroxypropyl guar derivatives bearing hydrophobic C10-C32 alkyl chains for use in water based paints and varnishes, wall coverings, adhesives and mortars. None of the above prior art, however, does disclose or suggest that highly substituted hydroxypropyl guar derivative comprising an unsubstituted linear or branched C6-C8 alkyl chain are particularly effective as water retention agents in compositions comprising a hydraulic binder.